To determine possible migratory responses to LPS of haemopoietic stem cells (CFUs) in LPS-nonresponsive (C3H/HeJ) mice, changes in the number of CFUs and granulocyte-macrophage progenitor cells (CFUc) after LPS-treatment where studied in splenectomized C3H/ HeJ mice. Splenectomy abrogated the decrease in bone marrow stem cells (CFUs and CFUc) induced by LPS in both LPS-responsive and nonresponsive mice. Recovery of bone marrow stem cells after irradiation was greatly enhanced by splenectomy in C3H/slc mice, but such enhancement was not observed in C3H/HeJ mice. These data suggest that the increase in the number of haemopoietic stem cells in the spleen in C3H/HeJ mice depends on the migration of haemopoietic stem cells from marrow to the spleen. Thus, the expansion of CFUs does not correlate with increase in the number of whole body haemopoietic stem cells. The migrationinducing activity of LPS seems to differ from its haemopoietic stimulatory activity, since the LPS-nonresponsive C3H/HeJ mice respond to the former activity of LPS.